I’m constantly running into multiple groups that address the same subject. It would be nice if I could combine those so I saw all the posts in one place. For example, all the forums that are discussing UAPs it would be cool to just look at a single thing that combined them all. 
This is something I think shouldn’t be specific to a single app. There’s an issue requesting this functionality on the Lemmy GitHub repo: https://github.com/LemmyNet/lemmy/issues/818
If you are interested, it would probably be good to leave an upvote (👍-reaction on the first post in the issue thread) for that issue and maybe a comment detailing your specific use-case and any ideas you might have for how it should work.